Photochromic material and rewritable optical recording medium

ABSTRACT

A photochromic material containing a copolymer of a spirobenzothiopyran derivative prepolymer and a liquid crystal prepolymer. In addition, a rewritable optical recording medium using the photochromic material. A rewritable optical recording medium obtained by coating a dispersed material, which is obtained by uniformly dispersing a spirobenzothiopyran derivative compound in a thermoplastic resin, on a substrate. A photochromic material capable of controlling a colored state and a decolorized state in a photon mode or a heat mode using an ultraviolet light source and a near-infrared light source. A rewritable optical recording medium capable of rewriting an information in a photon mode using a semiconductor laser.

This application is a division of application Ser. No. 07/655,999 filedon Feb. 14, 1991 now U.S. Pat. No. 5,164,287.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photochromic material used as variouskinds of recording material, displaying material, photographic printingmaterial, optical filter, photometer, masking material and the like. Inaddition, the present invention relates to an optical recording mediumused in for example an optical disk recording and regenerating apparatus(hereinafter referred to singly as recording and regeneratingapparatus), in particular to a rewritable optical recording mediumcapable of not only writing in and reading out informations by the useof laser beams but also rewriting informations.

2. Description of Related Art

An optical disk, which is a disk optical recording medium used in arecording and regenerating apparatus usually has recording function butan addition-type optical disk incapable of erasing recorded informationsafter recording them and a rewritable optical disk capable of erasingrecorded informations after recording them to record other informationsagain have been known. The recording and regenerating apparatus usingsuch the optical disks have been usually used as an outside memory of anelectronic computer.

The addition-type optical disk comprises a substrate made of glass orplastic and a metallic vapor deposited layer, such as a tellurium (Te)vapor deposited layer, having a low melting point and formed on thesubstrate as a recording medium. In a recording and regeneratingapparatus using this, the addition-type optical disk is rotated at ahigh speed to collect laser beams optically modulated depending uponinformations to be written in onto the metallic vapor deposited layer asa light absorptive layer, whereby subjecting the metallic vapordeposited layer to a thermal processing, in short the metallic vapordeposited layer is punched to write in the informations. In theaddition-type optical disk, the informations are written in the metallicvapor deposited layer by the thermal processing, as above described, sothat it is impossible to erase the informations recorded once.

On the other hand, in the rewritable optical disk, a photomagneticeffect, for example Kerr effect, and a crystalline-amorphous phasetransition have been utilized according to circumstances. For example,in a recording and regenerating apparatus using the rewritable opticaldisk utilizing the phase transition, the rewritable optical diskprovided with a tellurium family amorphous metallic layer formed as therecording medium on the surface of the substrate is rotated at a highspeed to collect laser beams optically modulated depending uponinformations to be written in onto the amorphous metallic layer, wherebyproducing the phase transition in the amorphous metallic layer to writethe informations in the amorphous metallic layer. And, in order to erasethe recorded informations, the phase transition is produced again in theamorphous metallic layer by the utilization of a heat of laser beams toreturn the amorphous metallic layer to the original condition (JapanesePatent Application Laid-Open No. Sho 62-165749 and Japanese PatentApplication Laid-Open No. Sho 62-165750). As above described, in case ofthe conventional rewritable optical disk, the writing in piles can beconducted by merely irradiating with lights and characteristics of anoncontact optical recording can be perfectly kept, which arepreferable.

A photochromic material composing such the rewritable optical recordingmedium is a material, which is colored by irradiating ultraviolet raysand decolorized by an irradiation of visible rays and a heat, and ismade by dispersing a photochromic compound in a solution or a highmolecular medium or carrying the photochromic compound on the highmolecular medium by a chemical bond. It has been known that inparticular the material made by carrying the photochromic compound onthe high molecular medium by a chemical bond can suppress an elusion ofthe photochromic compound from the medium and thus a heat stability of acolored body can be improved [refer to for example Kobunshi Ronbunshu.,33, 649 (1976)].

In addition, it has been disclosed in Mol. Cryst. Liq. Cryst., 1988,Vol. 155, pp 221-230 that a side chain-type high molecular liquidcrystal photochromic material exhibits a photochromism and athermophotochromism. On the other hand, sulfur-containing spiropyran hasbeen proposed as a photochromic compound having a sensitivity to a nearinfrared range [refer to for example Journal of Physical Chemistry.,Vol. 72, No. 3 (1968), pp. 997-1001]. This photochromic compound isreversibly colored and decolorized in a photon mode.

The photochromic material usable in the rewritable optical recordingmedium must be superior in repeated applicability of at leastcolorless→colored and colored→colorless and have an increasedcolor-changing speed when irradiated with a light not less than athreshold value but not be discolored when irradiated with a light lessthan the threshold value and superior in preservative stability underthe colorless condition and the colored condition, so that it is limitedby a spirobenzothiopyran derivative compound. However, the spiropyrancompound usually exhibits the following basic problems:

1. The sulfur-containing spiropyran is remarkably inferior in durabilityto the repeated application of coloring and decolorization and thecolored body is unstable to heat. In addition, in the case wheresulfur-containing spiropyran is used for the optical recording material,the recorded informations are broken by a reading-out light according tocircumstances because of the photon-mode recording.

2. The spiropyran compound is crystalline under the monomeric conditionand a coating is associated with it, so that a resolution power of theoptical recording medium (as for the optical disk, a diameter of 1 bitis about 1 μm) can not be sufficiently enhanced.

3. The colored spiropyran compound is usually thermally unstable to beisomerized into a colorless body, so that it is insufficient inpreservative stability.

SUMMARY OF THE INVENTION

The present inventors have found that it is advantageous to uniformlydisperse the spiropyran compound in a high molecular matrix in order tosolve the above-described problem 2 and it is advantageous for thisobject to uniformly disperse the sipropyran groups in the high molecularmatrix, as disclosed in Japanese Patent Application Laid-Open No. Hei1-294090. In addition, the present inventors have paid attention to thatit is necessary to give a thermal stability to the compound itself inorder to solve the above-described problem 3 and it had better introducehydroxyl groups into the spiropyran compound to utilize hydrogen boundsfor this object, as disclosed in Journal of the Chemical SocietyChemical Communication., p. 437 (1986). The present invention has beenachieved on the basis of such the circumstances in order to solve theabove described problems.

It is one object of the present invention to provide a photochromicmaterial improved in stability under the colored condition andremarkably easy to control the colored condition and decolorizedcondition thereof.

It is another object of the present invention to provide a photochromicmaterial capable of controlling the colored condition and decolorizedcondition thereof in a photon mode and a heat mode by the use of a lightsource (for example a semiconductor laser) of ultraviolet range andnear-infrared range.

It is a further object of the present invention to provide aphotochromic material capable of realizing a thermal stability by theuse of inexpensive organic materials.

It is a still further object of the present invention to provide arewritable optical recording medium capable of rewriting informations ina photon mode by the use of a semiconductor laser utilizing photochromiccharacteristics.

It is still another object of the present invention to provide arewritable optical recording medium capable of producing frominexpensive organic materials.

The photochromic material according to the present invention ischaracterized in containing a copolymer of a spirobenzothiopyranderivative prepolymer expressed by the following formula (I) and aliquid crystal prepolymer expressed by the following formula (II):##STR1## wherein R₁ is a hydrogen atom or a methyl group; R₂, R₃ arerespectively hydrogen atom, a hydroxyl group, a halogen atom, an aminogroup, a lower alkoxy group or an aryl group, R₂ is same as or differentfrom R₃ ; and m is an integer of 1 to 20. ##STR2## wherein R₄ is ahydrogen atom or a methyl group; R₅ is a nitryl group, a lower alkoxygroup, ##STR3## and n is an integer of 1 to 20.

In addition, the rewritable optical recording medium according to thepresent invention is characterized in using the above-describedphotochromic material having such the structure.

Further, another rewritable optical recording medium according to thepresent invention is characterized in producing by uniformly dispersinga spirobenzothiopyran derivative compound expressed by the followingformula (III) in a thermoplastic resin and coating the resultingdispersed material on a substrate. ##STR4## wherein t is an integer of 1to 20.

The above and further objects and features of the invention will morefully be apparent from the following detailed description withaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a copolymer used in a photochromicmaterial according to the present invention;

FIG. 2 is a graph showing a spectral change of the copolymer before andafter irradiating with ultraviolet rays;

FIG. 3 is a graph plotted in accordance with the Arrhenius' equation ofa reaction speed of a photo- and thermodecolorization of the copolymer;

FIG. 4 is a graph showing a change of the copolymer in colored conditionwith the lapse of time at a reduced intensity of ultraviolet rays;

FIG. 5 is a microscopic photograph showing dots recorded on a rewritableoptical recording medium according to the present invention;

FIG. 6 is a photograph showing a signal regenerated from the rewritableoptical recording medium according to the present invention;

FIG. 7 is a graph showing a spectral change of the rewritable opticalrecording medium according to the present invention before and afterirradiating with ultraviolet rays;

FIG. 8 is a microscopic photograph showing the rewritable opticalrecording medium according to the present invention after subjecting toa writing-in by a semiconductor laser beam; and

FIG. 9 is a graph showing results of a reading conducted by thesemiconductor laser beam for the rewritable optical recording mediumaccording to the present invention which has been subjected to thewriting-in.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The photochromic material according to the present invention contains acopolymer of a spirobenzothiopyran derivative prepolymer expressed bythe following formula (I) and a liquid crystal prepolymer expressed bythe following formula (II): ##STR5##

The spirobenzothiopyran derivative prepolymer [hereinafter referred toas prepolymer (A)] expressed by the general formula (I) is an ingredientfor realizing the photochromism. In the general formula (I), R₁ is ahydrogen atom or a methyl group; R₂, R₃ are a hydrogen atom, a hydroxylgroup, a halogen atom, an amino group, a lower alkoxy group or an arylgroup, respectively, R₂ being same as or different from R₃. Concretely,the lower alkoxy group and aryl group include a methoxy group, an ethoxygroup, a phenyl group, a tolyl group, a xylyl group, a naphthyl groupand the like. In addition, m is an integer of 1 to 20, preferable 1 to6. Because it is found from the experimental results that both aphotoresponse speed and a thermoresponse speed are increased with anincrease of m and if m is larger than 20, the photoresponse speed isincreased but also a thermal reaction-inhibiting effect by a mesogengroup is reduced, which is not becoming to the objects of the presentinvention.

Concrete examples of the prepolymer (A) include for example ##STR6##

A synthetic method of the prepolymer (A) is not specially limited. Itcan be synthesized according to for example a method of S. Arakawa etal. [refer to Chem. Lett., 1905 (1985)] and a method of J. Vorborgt etal. [refer to J. Polym. Sci. Polym. Chem. Ed., 12, 2511 (1974)].

The liquid crystal prepolymer [hereinafter referred to as prepolymer(B)] expressed by the general formula (II) is an ingredient forincreasing the thermal stability. In the general formula (II), R₄ is ahydrogen atom or a methyl group and R₅ is a nitryl group, a lower alkoxygroup, ##STR7## Concrete examples of the lower alkoxy group include amethoxy group, an ethoxy group and the like. In addition, n is aninteger of 1 to 20, preferably 1 to 6. If n is larger than 20, also thethermal reaction-inhibiting effect by the mesogen group for aphotochromic group is reduced in the same manner as for m, which is notbecoming to the objects of the present invention.

Concrete examples of the prepolymer (B) include ##STR8## and the like.

A synthetic method of the prepolymer (B) is not specially limited. Itcan be synthesized according to for example a method of M. Portugall etal. [refer to Macromol. Chem., 183, 2311 (1982)].

It is sufficient that the copolymer contained in the photochromicmaterial according to the present invention is obtained bycopolymerizing the prepolymer (A) and the prepolymer (B) in a ratio soas to exhibit a smectic condition or a nematic condition. In view ofthis point, it is preferable that the copolymer is obtained bycopolymerizing the prepolymer (A) and the prepolymer (B) in a ratio of 1to 50, in particular 10 to 30,: 100 parts by weight.

Furthermore, a length of spacer m and n in the prepolymer (A) and theprepolymer (B) may be different to each other but it is preferable inview of a matter that the length capable of putting the photochromicgroup between groups (hereinafter referred to as Lc) introduced into thecopolymer by the prepolymer (B) containing the mesogen group ispreferable that m is same as n.

Also a polymerization degree of the copolymer is not limited but thepolymerization degree gives an influence upon a temperature rangeexhibiting a liquid crystallinity, so that it is preferable that thepolymerization degree becoming to an object of the material used isselected.

A method of producing the copolymer is not specially limited. It can beproduced by the conventional polymerization methods.

The obtained copolymer is schematically shown in FIG. 1. Referring toFIG. 1, reference numeral 1 designates a main chain formed bycopolymerizing the prepolymer (A) and the prepolymer (B). The main chain1 comprises the mesogen group 3 introduced by the prepolymer (B), aphotochromic group I 4a, which is a spiropyran group introduced by theprepolymer (A), and a photochromic group II 4b introduced from thephotochromic group I 4a connected through spacers 2. In addition, Spdesignates a ring-closure stable body, Mer designating a ring-openingstable body, and Mer being thermally stabilized by regularly orientatingLc.

The photochromic material containing a polymer according to the presentinvention is greatestly characterized in being capable of giving athreshold value to the photoisomerization reaction, suppressing thethermoisomerization reaction, and thus achieving the appointed objects.In short, as to the coloring and decolorization of the photochromicmaterial according to the present invention, it goes without saying thatthe colored condition can be obtained by irradiating with ultravioletrays and the decolorized condition can be obtained by irradiating withstrong near-infrared rays and this transition can be repeated manytimes. The colored condition is difficulty returned to the decolorizedcondition by irradiating with relatively weak near-infrared rays andvisible rays.

The preferred embodiments of the photochromic material according to thepresent invention will be below described.

The spirobenzothiopyran derivative prepolymer [prepolymer (A)] issynthesized in the following manner.

In short, 5-nitrothiosalicylaldehyde is synthesized in the followingprocedures according to the method of S. Arakawa et al. ##STR9##

Then, spirobenzothiopyran derivative is synthesized in the followingprocedures according to the method of J. Vorborgt et al. ##STR10##wherein k is 2, 3 or 6.

The liquid crystal prepolymer [prepolymer (B)] is synthesized in thefollowing procedures according to the method of M. Portugall et al.##STR11## wherein p is 2, 3, 4 or 6; R₆ is CN or OCH₃.

One example of the identification results of the prepolymer (A) is shownin Table 1.

                  TABLE 1                                                         ______________________________________                                                    Results of the elemental analysis (%)                             Compound  k            C        H     N                                       ______________________________________                                        Prepolymer                                                                              2       calcd.   65.39  5.25  6.63                                  (A)               found    66.25  5.09  6.46                                  Prepolymer                                                                              6       calcd.   67.76  6.32  5.85                                  (A)               found    68.17  6.48  5.88                                  ______________________________________                                    

The prepolymer (A) of 0.2 mM and the prepolymer (B) of 2 mM, which havebeen obtained in this, are copolymerized in toluene of 10 ml with usingAIBN of 2 mol % based on a total quantity of the prepolymer (A) and theprepolymer (B) as an initiator to obtain first to fifth copolymers shownin Table 2.

                  TABLE 2                                                         ______________________________________                                                                         Polymerization                                           k      p      R.sub.6                                                                              degree                                       ______________________________________                                        First copolymer                                                                           2      2      --OCH.sub.3                                                                          20,000                                       Second copolymer                                                                          2      2      --CN   15,000                                       Third copolymer                                                                           3      3      --CN   20,000                                       Fourth copolymer                                                                          6      6      --OCH.sub.3                                                                          18,000                                       Fifth copolymer                                                                           6      6      --CN   20,000                                       ______________________________________                                    

The obtained respective copolymers (photochromic materials) of 3 mg areplaced on a glass plate to be molten at 100° C. and then covered with acover glass to be uniformly spread, whereby preparing test pieces of 10μm thick.

The prepared 5 test pieces are irradiated with ultraviolet rays(obtained by passing a light from a xenon lamp of 500 W through anultraviolet filter) for 1 minute and a change of each test piece incolor is investigated immediately after the irradiation and afterstoring in a darkroom for 3 months. The test pieces are yellow beforethe irradiation. The test pieces are all colored in green immediatelyafter the irradiation of ultraviolet rays. In addition, the test piecesall hardly exhibit a change of color before and after storing for 3months in a darkroom.

The test piece prepared from the first copolymer (first test piece) isinvestigated on an absorption spectrum at 25° C. before and after theirradiation of ultraviolet rays by using an absorption meter. Theresults are shown in FIG. 2. FIG. 2(a) shows an absorption spectrum ofthe test piece before the irradiation of ultraviolet rays. FIG. 2(b)shows an absorption spectrum of the test piece after the irradiation.

It is found from FIG. 2 that this material exhibits a photochromism anda colored body has an absorption peak at 705 nm and an absorption in anear-infrared range. In addition, the λ_(MAX) of the spirobenzothiopyranderivative prepolymer is 680 nm. It is found that the absorption peak ofthe photochromic material according to the present invention is shiftedto a long wavelength side by 25 nm.

In addition, upon irradiating the first test piece, which has beenirradiated with ultraviolet rays, with visible rays, the absorptionspectrum of the first test piece is returned to that before theirradiation of ultraviolet rays and this can be repeated many times.

A reaction speed [K] in the photodecolorization and thethermodecolorization of the first test piece is determined from a changeof a light having a wavelength of 680 nm, which had passed through thecolored body under the irradiation of visible rays, with a lapse oftime. The Arrhenius' plot of the photodecolorization reaction processand the thermodecolorization reaction process is shown in FIG. 3. FIG.3(c) shows the photodecolorization reaction process and FIGS. 3(d), (e)show the thermodecolorization reaction process.

It is found from FIG. 3 that the first copolymer is nematic at 80° C. ormore and glassy nematic at temperatures lower than 80° C. and thethermodecolorization speed of the process (d) at a room temperature isremarkably reduced. It seems that the thermodecolorization process (e)includes a process which is not stabilized by Lc.

In addition, a change of an absorption of the light having a wavelengthof 680 nm is measured when an intensity of ultraviolet rays is reducedto 5% of the above described one. The results are shown in FIG. 4. It isfound from FIG. 4 that the colored body can not be obtained ifultraviolet rays are weak.

This is similar also to an intensity of visible rays in the obtainmentof a colorless body.

It is found from the above-described results that the photochromicmaterial according to the present invention exhibits a photochromism andthe colored condition is thermally stable and additionally the coloredcondition and decolorized condition can be controlled by an intensity oflight.

The first copolymer (200 mg) is dissolved in tetrahydrofuran (2 ml) tobe applied to a non-grooved glass plate having a diameter of 130 mm bythe spin coating method. A film-thickness is about 1 μm after drying.The resulting film is annealed for 10 minutes at 80° C. and thenaluminum is vacuum vapor coated (about 550 A) on the film as areflecting film to obtain an optical recording medium.

As to a recording method, at first the optical recording medium isirradiated with ultraviolet rays using a black light to color all overthe surface thereof and a signal is recorded (decolorized) on theoptical recording medium by a semiconductor laser. The recordingconditions are as follows: a wavelength: 780 nm; a power: 14 to 20 mW; abeam half band width: 1.3 μm; a linear velocity: 3.85 m/s; a frequencyof carrier: 100 kHz to 1 MHz; a duty ratio: 50%; and a regeneratingpower: 1 mW.

FIG. 5 is a microscopic photograph showing a recorded dot recorded by apower of 14 to 20 mW and a sweeping signal of 100 kHz to 1 MHz. A widthbetween recording lines is about 1 μm. A portion expressed by a lightwhite line on a left side of the photograph is written-in at 14 mW and100 kHz and a right portion is recorded at 20 mW. The record at 20 mWand 1 MHz exhibites a C/N ratio of 27 dB. In addition, it is found thatthe record could not be achieved under the above described conditions ata recording power of 14 mW or less and the recording power has athreshold value. That is to say, a recording sensitivity is 350 to 500mJ/cm².

Furthermore, as to the regeneration, it is found that the regenerationcan be continuously repeated about 6,000 times at a regenerating powerof 1 mW. FIG. 6 is a photograph showing a regenerated signal.

Besides, it is confirmed also that this record can be erased byirradiating with ultraviolet rays using the black light again andrepeatedly recorded by means of the semiconductor laser. In addition, astability in preservation of the record already clears 150 days at aroom temperature.

It can be found from the above described results of recording test thatthe rewritable optical recording medium, to which the photochromismaccording to the preferred embodiments of the present invention isapplied, is superior.

In addition, the regenerating stability can be still more increasedunder the thermally mild conditions. Furthermore, the substrate usedhere is provided with no groove and no high-refractive index layer butit is sure that if the substrate is provided with them, the C/N ratioand the recording sensitivity are still more improved.

As above described, since the photochromic material according to thepresent invention has a high stability of colored condition and gives athreshold value to the photoisomerization reaction, the coloredcondition and the decolorized condition can be very easily controlled.Accordingly, the optical recording medium using the photochromicmaterial according to the present invention is superior. In addition,the photochromic material according to the present invention can be usedas various kinds of recording memorizing material, the displayingmaterial for the display and the like, the photographic printingmaterial, the optical filter, the photometer, the masking material andthe like.

Next, the rewritable optical recording medium according to anotherpreferred embodiment of the present invention will be described.

In the rewritable optical recording medium according to this preferredembodiment, a spirobenzothiopyran derivative compound expressed by thefollowing formula (III) is used as the photochromic material. ##STR12##

In the teneral formula (III), t is an integer of 1 to 20, preferablyabout 1 to 6. Because it is found from the experimental results thatboth a photoresponse speed and a thermoreactive speed are increased withan increase of t and if t is larger than 20, the photoresponse speed isincreased but also a thermal reaction-inhibiting effect by athermoplastic resin used together is reduced to make the achievement ofthe objects of the present invention difficult.

Of such the compound expressed by the general formula (III), those, inwhich t is 1, 2, 3, 4 or 6 are preferably used.

In addition, the compound expressed by the general formula (III) hasbeen found as the optimum one of spiropyran compounds expressed by thefollowing formula (IV) superior in stability of the record preservationand repeated use and showing a relatively high photoresponse speed inview of photorecording use. ##STR13## wherein at least one of R₇, R₈ andR₉ has a hydroxyl group; others are any one of a hydrogen atom, ahalogen group, a nitro group, an amino group, an alkyl group, an alkoxygroup and an aryl group.

A synthetic method of the compound expressed by the general formula(III) is not specially limited. It can be synthesized according to forexample a method of S. Arakawa et al. [refer to Chem. Lett., 1905(1985)] and a method of J. Vorborgt et al. [refer to J. Polym. Sci.Polym. Chem. Ed., 12, 2511 (1974)].

The thermoplastic resin used in this preferred embodiment is aningredient used as a matrix and its concrete examples include styreneresins, such as polystyrene, acrylic resins, such as polymethylmethacrylate, polyester resins, polyamide resins, polyethylene,polypropylene, polyvinylidene fluoride, polyvinylidene chloride,polyvinyl chloride, ethylene-vinyl acetate copolymers, vinylchloride-vinyl acetate copolymer, styrene-acryl copolymers,styrene-butadiene copolymers, styrene-vinylidene chloride copolymers,styrene-vinyl chloride copolymers, epoxy resins, polycarbonate resins,mixtures thereof and the like but it is not limited by them.

It is preferable that a glass transition temperature (Tg) of thethermoplastic resin is 50° C. or more. Because it is necessary that theTg is at least a temperature of natural environment and thethermoplastic resin is used under a glassy state of high moleculewithout fail, that is a segmental movement is inhibited, and, if thesegmental movement is released, the thermoisomerization reaction of thecompound expressed by the general formula (III) [hereinafter referred toas compound (III)] is promoted to make the achievement of the desiredends impossible.

The rewritable optical recording medium according to this preferredembodiment has a construction that a dispersed material with thecompound (III) uniformly dispersed in the thermoplastic resin[hereinafter referred to as dispersed material (III)] is coated on asubstrate.

It is preferable that the compound (III) is used in a quantity of 10 to200 parts in usual, in particular 10 to 50 parts, based on 100 parts ofthe thermoplastic resin.

A method of uniformly dispersing is not specially limited. It issufficient that for example the thermoplastic resin and the compound(III) are dissolved in solvents such as toluene, xylene andtetrahydrofuran.

Every light-transmissive or light-reflecting substrate, which has beengenerally used for the rewritable optical recording medium, made of forexample glass, ceramics, metals, plastics and the like can be used asthe substrate.

Also a method of coating the dispersed material (III) on the substrateis not specially limited. For example, a method, in which thethermoplastic resin and the compound (III) are dissolved in the solventsand the resulting solution is coated on the substrate by the sprayingmethod and the like followed by drying, can be used.

It is preferable in view of using the recording and regenerating methodby a laser beam that a film-thickness of a coating formed in such themanner is 0.3 to 3.0 μm as usual.

In the rewritable optical recording medium according to this preferredembodiment, for example an aluminum film of about 0.2 to 1 μm thick maybe formed on the coating by the usual methods such as vacuum vaporcoating method.

The record of informations into the rewritable optical recording mediumaccording to such the preferred embodiment can be achieved by at firstinitializing the medium by means of ultraviolet rays having a wavelengthof 380 nm at a recording sensitivity of about 1 J/cm² and thenirradiating the recorded portion with a collected strong visible rayhaving a power of about 3 to 20 mW to make the recorded portioncolorless.

And, the recorded informations can be regenerated by discriminating anexistence of coloring in each bit by a relatively weak light having awavelength of about 600 to 800 nm and a power of about 0.2 to 0.8 mW.The discrimination of an existence of coloring may be conducted by anyone of a transmitted light and a reflected light.

In addition, a light source for recording, regenerating or erasing theinformations includes a laser, a mercury lamp, a xenon lamp, a metalhalide lamp, a halogen lamp, a tungsten lamp and the like. An opticalfilter may be used together.

The concrete production of the rewritable optical recording mediumaccording to this preferred embodiment will be below described.

5-nitrothiosalicylaldehyde is synthesized in the following proceduresaccording to the method of S. Arakawa et al. ##STR14##

Then, the spirobenzothiopyran derivative compound is synthesized in thefollowing procedures according to the method of J. Vorbogt et al.##STR15## wherein x is 2 or 6.

The identification data (results of the elemental analysis) of theobtained spirobenzothiopyran derivative compound having x of 2(hereinafter referred to as BTP-2) and the obtained spirobenzothiopyranderivative compound having x of 6 (hereinafter referred to as BTP-6) areshown in Table 3.

                  TABLE 3                                                         ______________________________________                                                  Results of the elemental analysis (%)                               Compound          C        H        N                                         ______________________________________                                        BTP-2       calcd.    65.20    5.47   7.60                                                found     61.69    4.59   5.06                                    BTP-6       calcd.    67.90    6.65   6.60                                                found     66.14    6.06   6.37                                    ______________________________________                                    

Polymethyl methacrylate (hereinafter referred to as PMMA) as thethermoplastic resin of 1 g or polystyrene as the thermoplastic resin of1 g and BTP-2 of 285 mg or BTP-6 of 570 mg are dissolved in xylene of 8g to prepare 4 kinds of composition. In addition, a composition composedof PMMA of 1 g, BTP-2 of 285 mg and xylene of 8 g is hereinafterreferred to as BTP-2/PMMA.

The obtained compositions are coated on a glass substrate by the spraycoating so that the film-thickness may be 1.0 μm after drying and thenan aluminum layer of 0.5 μm thick is formed on the film by the vacuumvapor coating method to obtain rewritable optical recording mediums.

For the obtained rewritable optical recording mediums the measurement ofa visible absorption spectrum, the microscopic observation and thereading and the reading test are conducted. The results are shown inFIGS. 7 to 9.

FIG. 7 is a graph showing a change of the rewritable optical recordingmedium using BTP-2/PMMA in visible absorption spectrum before and afterirradiating with ultraviolet rays. The visible absorption spectrum ismeasured before [(a) in FIG. 7] and after [(b) in FIG. 7] irradiatingthe rewritable optical recording medium using BTP-2/PMMA withultraviolet rays having a wavelength of 380 nm. It is found from FIG. 7that this optical recording medium exhibits the photochromism and thecolored body has the absorption peak in the vicinity of 690 nm andabsorbs also a semiconductor laser beam having a wavelength of 780 nm.

FIG. 8 is a microscopic photograph of the rewritable optical recordingmedium using BTP-2/PMMA when written-in by a semiconductor laser beam.This rewritable optical recording medium is irradiated with ultravioletrays having a wavelength of 380 nm and a power of 100 μW/cm² for 20seconds to be initialized and then the writing-in is conducted by asemiconductor laser beam (780 nm) under the conditions that a power is20 mW, a frequency being 100 kHz, and a rotational frequency being 600r.p.m. followed by the microscopic observation. It is found from FIG. 8that the colored body is black and the portions, which have been turnedinto colorless bodies by the irradiation of light, are white, whichshows the achievement of the writing-in.

FIG. 9 is a graph showing results of the reading by a semiconductorlaser beam after the writing-in for the rewritable optical recordingmedium using BTP-2/PMMA. In FIG. 9, an axis of ordinate shows areflectance (%) and an axis of abscissa shows a time (μs). For thisrewritable optical recording medium the writing-in is conducted underthe same conditions as in the above-described microscopic observationand then irradiated with a semiconductor laser beam having a power of0.8 mW and a wavelength of 780 nm at a rotational frequency of 600r.p.m. to measure a reflectance of the semiconductor laser beam, wherebyconducting the reading. It is found from FIG. 9 that the reflectance inthe written-in portions is increased even after the reading 100 times,which shows the stabilized reading.

It is found from the above-described results that also the rewritableoptical recording medium according to this preferred embodiment hassuperior characteristics.

As this invention may be embodied in several forms without departingfrom the spirit of essential characteristics thereof, the presentembodiment is therefore illustrative and not restrictive, since thescope of the invention is defined by the appended claims rather than bythe description preceding them, and all changes that fall within themetes and bounds of the claims, or equivalence of such metes and boundsthereof are therefore intended to be embraced by the claims.

What is claimed is:
 1. A rewritable optical recording medium comprisinga photochromic material containing a copolymer of a spirobenzothiopyranderivative prepolymer expressed by the following formula (I) and aliquid crystal prepolymer expressed by the following formula (II):##STR16## wherein R₁ is selected from the group consisting of a hydrogenatom and a methyl group;R₂ is selected from the group consisting of ahydrogen atom, a hydroxyl group, a halogen atom, an amino group, a loweralkoxy group and an aryl group; R₃ is selected from the group consistingof a hydrogen atom, a hydroxyl group, a halogen atom, an amino group, alower alkoxy group and an aryl group; and m is an integer of 1 to 20;##STR17## wherein R₄ is selected from the group consisting of a hydrogenatom and a methyl group; R₅ is selected from the group consisting of anitryl group, a lower alkoxy group, ##STR18## and n is an integer of 1to
 20. 2. A rewritable optical recording medium as set forth in claim 1wherein m is an integer of 1 to
 6. 3. A rewritable optical recordingmedium as set forth in claim 1 wherein m is 2 and R₁, R₂ and R₃ are alla hydrogen atom.
 4. A rewritable optical recording medium as set forthin claim 1 wherein m is 6 and R₁, R₂ and R₃ are all a hydrogen atom. 5.A rewritable optical recording medium as set forth in claim 1 wherein nis an integer of 1 to
 6. 6. A rewritable optical recording medium as setforth in claim 1 wherein n is 2, R₄ is a hydrogen atom and R₅ is --OCH₃.7. A rewritable optical recording medium as set forth in claim 1 whereinn is 2, R₄ is a hydrogen atom and R₅ is --CN.
 8. A rewritable opticalrecording medium as set forth in claim 1 wherein n is 6, R₄ is ahydrogen atom and R₅ is --OCH₃.
 9. A rewritable optical recording mediumas set forth in claim 1 wherein n is 6, R₄ is a hydrogen atom and R₅ is--CN.
 10. A rewritable optical recording medium as set forth in claim 1wherein m and n have the same value.
 11. A rewritable optical recordingmedium as set forth in claim 1 wherein said spirobenzothiopyranderivative prepolymer is used in a quantity of 1 to 50 parts by weightbased on 100 parts by weight of said liquid crystal prepolymer.